Skorpion running rich

[ralf]

Help !! My Skorpion is running hopelessly rich. It doesn't seem to matter what I do with the mixture screw, it still runs rich, the spark plug always being black and very sooty. It runs fine with a cleaned spark plug until it fouls then the idle becomes lumpy and it then won't start from cold.

The petrol level in the bowl is correct and it's got a No.48 pilot jet which is as per the specification. I've also fitted a new mixture screw which is 2.5 turns out which seems to be as good as it gets, all to no avail. Is it worth trying a new pilot jet?

The bikes done 37000KM and looking back it's always been a reluctant cold starter if left for anything more than about a week.

Any help appreciated.
Thanks,
Neil.

[DAVID THOMPSON]

all 3 of my MZ bikes have been a reluctant cold starter if left for anything more than about a week. the RT125 is bad about it
if ran on fuel with ethanol in it
the lean mix to comply with pollution rules and the poor gas is the real problem
dave

[MZ in Oz]

Choke/enriching plunger most likely stuck. Remove and polish up with fine wet/dry abrasive and smear a little silicon grease on it.
Also check the small diaphragm on left carb to see if it has any holes.

[edfmaniac]

Mixture screw doesn't effect spark plug condition for the most part. It's only in the equation when the bike is at idle. If you want to change the plug color, you are going to have to start going leaner on the main jets which are notoriously rich from the factory, turbo rich as my dyno tech put it. Mains are rich and pilot jet is too lean. Here's what worked well for me, Keihin 135 and 155 for the mains and a Mikuni TM28/486 #40 or #38 on the pilot. You'll be pleasantly surprised by how much more responsive the bike is when it's jetted correctly.

[ralf]

Thanks very much for the advice so far. I forgot to say that the choke plunger was free in the carb body, the O ring looking good, the plunger face seal looks good and there is a bit of slack present in the cable . Fuel leaking past the choke plunger was my initial main suspect but it looks unlikely although it can't be completely dismissed. I will follow up on the other suggestions and report back.
Thanks.

[edfmaniac]

The easiest way to see if the lean pilot jet is causing the starting problem is to unscrew mixture setting as far as possible without falling out. That will give you the richest setting the jet is capable of producing although you shouldn't have to go to these extremes if you have the correct jet. Best method for me was to close the mixture screw all the way being sure not to overtighten, then unscrew until it falls out, counting the number of turns. If it falls out at 9 turns, you'll know that 7 1/2 is about as far out as you can safely run it. If that fixes your starting problems, you know it's the jetting without having to do any disassembly.

[ralf]

Thanks everybody. It was, as one of the posts suggested, a problem with the so called "coasting enricher" valve on the primary carburettor. Striped and reassembled all is now well in the MZ garage again.
Thanks to everybody who offered suggestions.

[breakwellmz]

Thank goodness for that well done mate!
I`ll be able to sleep tonight. ...................................................
.................There isn`t one of those on my Mastiff is there?